1. Field of the Invention
The present invention relates to an automatic transmission.
2. Discussion of the Background
In automatic transmissions for vehicles, increasingly large numbers of gear speeds have been employed to improve the running performance and drivability of vehicles. An automatic transmission generally includes planetary gear sets and engagement assemblies including a clutch and a brake. The automatic transmission performs gear changes by changing power transmission routes from one to another by using the engagement assemblies. Thus, the number of gear speeds can be increased by increasing the number of planetary gear sets or engagement assemblies. For example, Japanese Unexamined Patent Application Publication No. 2005-273768 discloses an automatic transmission that achieves eight forward speeds by using three planetary gear sets and six engagement assemblies.
When an engagement assembly, such as a wet clutch, is disengaged (or released), the engagement assembly may cause a frictional loss due to what is called drag. As the number of engagement assemblies is increased to increase the number of gear speeds, the number of engagement assemblies that are disengaged at each gear speed increases accordingly. For example, in the automatic transmission described in Japanese Unexamined Patent Application Publication No. 2005-273768, at each gear speed, two engagement assemblies are engaged while the remaining four engagement assemblies are disengaged. As more engagement assemblies become disengaged, more frictional losses may occur.